1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display device, and in particular to a method of driving a surface mods LCD such as a pi-cell device. It also relates to a liquid crystal display device.
2. Description of the Related Art
The pi-cell (otherwise known as an xe2x80x9coptically compensated birefringent devicexe2x80x9d or OCB) is described in xe2x80x9cMol. Cryst. Liq. Cryst.xe2x80x9d, 1984, Vol 113, p329-339, and In U.S. Pat. No. 4,635,051. The structure of a pi-cell is schematically illustrated in FIG. 1(a). The device comprises transparent substrates 2, 2xe2x80x2 on which are disposed alignment layers 3, 3xe2x80x2. A layer of nematic liquid crystal 1 is disposed between the substrates 2, 2xe2x80x2.
The alignment layers 3, 3xe2x80x2 create parallel alignment of the liquid crystal molecules in the liquid crystal layer 1 at its boundaries with the alignment layers 3, 3xe2x80x2. This can be achieved by using parallel-rubbed polyamide alignment layers. The pretilt induced by the alignment layers is generally under 45xc2x0 and is typically in the range 2xc2x0 to 10xc2x0.
Addressing electrodes (not shown) are provided on the substrates 2, 2xe2x80x2, so that an electric field can be applied to selected areas of the liquid crystal layer.
FIG. 1(a) shown the device when no electric field is applied across the liquid crystal layer. The liquid crystal is in an H-state (homogenous state, also known as a splay state), in which the liquid crystal molecules in the centre or the liquid crystal layer are substantially parallel to the substrates. The short lines in the figure represent the director of the liquid crystal molecules.
When an electric field greater than a threshold value is applied across the liquid crystal layer, the liquid crystal molecules adopt a V-state (or a bend state). In this state, the liquid crystal molecules in the centre of the liquid crystal layer are substantially perpendicular to the substrates. FIG. 1(b) shows a first V-state which occurs at a low applied voltage across the liquid crystal layer, and FIG. 1(c) shows a second V-state which occurs when a higher voltage is applied across the liquid crystal layer. The pi-cell is operated by switching the liquid crystal layer between the first, low voltage V-state and the second, higher voltage V-state.
As can be seen by comparing FIGS. 1(b) and 1(c), modulating the electric field applied across the liquid crystal layer causes the director of liquid crystal molecules close to the substrates to be reoriented, while the directors of liquid crystal molecules in the central region (in the thickness direction) of the liquid crystal layer remain substantially perpendicular to the plane of the substrates. For this reason, the pi-call is known as a surface mode device, and operates with the director in a bend state. Surface mode LCDs are disclosed in xe2x80x9cSov. J. Quant. Electronicsxe2x80x9d, 1973, Vol 3, page 78-79.
Surface mode devices have the advantage that they tend to exhibit a more rapid electro-optic response than most other nematic liquid crystal devices. Since liquid crystals have optically anisotropic properties, placing a pi-cell between two polarisers and varying the voltage applied across the liquid crystal layer causes a variation in the optical transmission, and this makes possible the formation of a light modulating device. For example, a pi-cell may be placed between linear polarisers whose transmission axis are crossed with one another and are at 45xc2x0 to the optic axis of the liquid crystal layer. Alternatively, a pi-cell may be arranged to operate in a reflective mode using only a single polariser.
One known problem is that when the electric field across the liquid crystal layer is reduced below the threshold voltage for the transition from the H-state to the V-state, the directors of the liquid crystal molecules adopt the H-state, or splay-state, shown in FIG. 1(a). The transition from this 0V splay-state to the required operating state is slow, and when a display device that incorporates a pi-cell is turned on there is a delay before the required operating state forms.
One attempt to overcome this problem, often referred to as the xe2x80x9cnucleation problemxe2x80x9d, is described in U.S. Pat. No. 4,566,756. This patent addresses the nucleation problem by adding a chiral material to the liquid crystal, so that the liquid crystal director adopts a 180xc2x0 twist state under a condition of no applied voltage. This is shown in FIG. 2. In contrast, the device illustrated in FIGS. 1(a) to 1(c) has a 0xc2x0 twist angle.
When a sufficiently high voltage (typically around 3V or greater) is applied across a chiral doped pi-cell having a 180xc2x0 twist angle it will exhibit an essentially identical director bend state to a non-doped, 0xc2x0 twist pi-cell. In fact, a 180xc2x0 twist p1-cell does not reach a true bend state (that is, a state where the director in the central region of the liquid crystal layer is perpendicular to the substrates) at any finite voltage. However, at high applied voltages the liquid crystal state of a 180xc2x0 twist pi-cell is a good approximation to a band state. In contrast, at low voltages (typically around 3V or below), a chiral doped pi-cell and a non-doped pi-cell will differ significantly in their operating properties.
Although a chiral doped 180xc2x0 twist state pi-cell of the type disclosed in U.S. Pat. No. 4,566,756 overcomes the nucleation problem, it has the disadvantage that the voltage applied over the pi-cell must remain above a certain level (typically around 3V or above) to ensure that the device operates in a surface mode. For example, if the voltage applied to such a device were switched between 0V and 10V the director of liquid crystal molecules in the centre of the liquid crystal layer would switch between 0xc2x0 (that is, parallel to the substrates) and substantially 90xc2x0 (that is, substantially perpendicular to the plane of the substrates). The device clearly would not then perform as a surface mode switching device, and thus would not achieve the high speed of operation expected for a surface mode device.
In order for the pi-cell disclosed in U.S. Pat. No. 4,566,756 to function as a surface mode device and thus retain the short switching time associated with a surface mode device, it is necessary for the voltage applied across the pi-cell not to fall below around 3V. This requirement means that the full range of optical response of liquid crystal is not available. In particular, bright regions of the optical response curve may not be available.
FIG. 3 is a schematic illustration of the relationship between equilibrium optical transmissivity of a chiral doped 180xc2x0 twist pi-cell against the voltage applied across the cell. Curves of this sort are routinely used to determine the voltages that should be applied across a liquid crystal layer in a display device. The inserts in FIG. 3 schematically show the director configuration of the liquid crystal molecules for various applied voltages.
When voltage D its applied across the liquid crystal layer, the display has a low transmissivity, and the director of liquid crystal molecules in the centre of the liquid crystal layer is predominantly perpendicular to the cell substrates. That is, the liquid crystal state is a close approximation to a bend state. In contrast, when a voltage close to zero (voltage A) is applied across the liquid crystal layer, it adopts a 180xc2x0 twist state, and the director of the molecules in the centre of the liquid crystal layer is parallel to the plane of the substrates. At intermediate voltages, the liquid crystal undergoes a complex variation in its optical transmissivity as the molecules re-orient themselves under the action of the applied voltage.
It will be seen that the transmissivity of the pi-cell only slowly tends towards zero as the applied voltage increases. It may therefore be desirable to incorporate a fixed retarder in a pi-cell, so that zero transmissivity is obtained at a finite applied voltage.
From a consideration of just the transmissivity of the liquid crystal device, it might appear most favourable to drive the liquid crystal device between a voltage such as D, where the liquid crystal device has a low transmissivity, and a voltage such as B where the liquid crystal device has a high, close to 100%, transmissivity in order to maximise the contrast. However, when voltage B is applied across the liquid crystal layer the orientation state of the liquid crystal molecules is not close to a bend state, since the director of liquid crystal molecules in the centre of the liquid crystal layer deviates significantly from the direction perpendicular to the substrates of the device. Thus, if the device were operated by switching the applied voltage between B and D, the device would not operate as a surface mode device, and would not achieve the rapid switching of a surface mode device. In order to operate the device in the surface mode it is necessary to vary the voltage between C and D to ensure that the central director remains substantially perpendicular to the substrates. Although operating the device in this way achieves rapid switching, it has the disadvantage that the maximum transmissivity of the display device that can be obtained is well below 100%, and this reduces the brightness and contrast of the display.
EP-A-0 149 899 discloses a method of driving a ferroelectric liquid crystal display device. A direct current voltage is applied across the liquid crystal layer to provide the liquid crystal layer with a first transmissivity. The transmissivity of the ferroelectric liquid crystal layer is then altered, by applying an alternating current voltage across the liquid crystal layer. The alternating current voltage has a different magnitude to the previously-applied direct current voltage.
U.S. Pat. No. 4,773,716 discloses a method of driving a ferroelectric liquid crystal display device. Two or more high voltage pulses are applied to put the ferroelectric liquid crystal layer into a particular state and, once this has been done, pulses having a lower voltage are applied to maintain the ferroelectric liquid crystal in the selected state for the remainder of a frame.
A first aspect of the present invention provides a method of driving a surface-mode liquid crystal display device including a liquid crystal layer having a non-zero twist angle, the method comprising the stop of: applying a first voltage having a first magnitude across the liquid crystal layer to put the liquid crystal layer into a first liquid crystal state; the method being characterised in that it further comprises changing the magnitude of the voltage applied across the liquid crystal layer while the brightness of the liquid crystal display device is greater than the equilibrium brightness value associated with the first liquid crystal state.
It has been found that if the voltage applied across a liquid crystal layer of a surface mode liquid crystal display device having a non-zero twist angle liquid crystal layer is changed the brightness does not change monotonically. For example if the applied voltage is reduced in order to increase the brightness of the liquid crystal device, the brightness of the liquid crystal device does not rise monotonically to the new equilibrium value. Instead, the brightness rises rapidly to a value that is greater than the equilibrium value, and then slowly decreased to the equilibrium value. If the voltage applied across the liquid crystal layer is subsequently changed while the brightness of the liquid crystal device is greater than its equilibrium value, the brightness of the display can be increased. In the case of a transmissive display device the transmissivity is increased and in the case of a reflective device the reflectivity is increased.
The step of changing the magnitude of the voltage applied across the liquid crystal layer may be performed before the director of liquid crystal molecules in the centre in a thickness direction of the liquid crystal layer has substantially reached its equilibrium orientation associated with the first liquid crystal state.
The step of changing the magnitude of the voltage applied across the liquid crystal layer may be carried out after no more than 50% of the time required for the brightness of the liquid crystal device to reach substantially the equilibrium value associated with the first liquid crystal state. The magnitude of the voltage applied across the liquid crystal layer may be changed after approximately 32% of the time required for the brightness of the liquid crystal device to reach substantially the equilibrium value associated with the first liquid crystal state. The magnitude of the voltage applied across the liquid crystal layer may be changed no later than approximately 16 msec after the step of applying the first voltage. In these cases, the voltage applied across the liquid crystal layer is changed before the brightness of the liquid crystal device has decayed to its equilibrium value, so that the brightness of the display is increased.
The method may further comprise the step of putting the liquid crystal layer into a known liquid crystal state before performing the step of applying the first voltage. The step of putting the liquid crystal layer into a known liquid crystal state may comprise applying a blanking voltage to the liquid crystal layer. This enables reproducible grey scale levels to be obtained.
The magnitude of the blanking voltage may be dependent on the temperature of the liquid crystal layer. As the temperature of the liquid crystal layer varies, the viscous properties of the liquid crystal layer will change. Varying the magnitude of the blanking voltage with the temperature of the liquid crystal layer will compensate for any changes in the viscous properties of the liquid crystal layer.
The liquid crystal layer may contain a chiral dopant. The pitch induced by the chiral dopant in the liquid crystal may decrease with temperature. This is an alternative way to counteract the increase in viscosity of a liquid crystal material that generally occurs as the temperature rises.
The chiral dopant may induce a twist in the director of the liquid crystal molecules having a pitch p such that d/p=0.25, where d is the thickness of the liquid crystal layer. Alternatively, the chiral dopant may induce a twist in the director of the liquid crystal molecules having a pitch p such that d/pxe2x89xa70.25, where d is the thickness of the liquid crystal layer. The chiral dopant may induce a twist in the director of the liquid crystal such that 0.25xe2x89xa6d/pxe2x89xa60.5. A lower d/p ratio results in less temporal variation in the brightness of the liquid crystal display device once the peak brightness has been reached. For a relatively long frame time, use of a liquid crystal layer with a low d/p ratio will minimise variations in brightness over the duration of the frame.
The twist angle of the liquid crystal layer may be substantially 180xc2x0.
The liquid crystal display device may be an active matrix display device comprising an array of pixels, each pixel being defined by a corresponding pixel electrode, M strobe electrodes and N signal electrodes, and the method may comprise, in a frame, the steps of:
(a) applying a blanking voltage to each of the pixels;
(b) applying a respective signal voltage corresponding to desired image data to each of the pixels;
(c) allowing the liquid crystal molecules to switch states; and
(d) displaying the image.
The step (a) may comprise applying the blanking voltage substantially simultaneously to all pixels. Alternatively, the step (a) may comprise applying the blanking voltage to pixels associated with each strobe electrode in sequence.
The step (b) may comprise applying the signal voltages to pixels associated with each strobe electrode in the sequence 1, 3, 5 . . . M, Mxe2x88x921, Mxe2x88x923, . . . 6, 4, 2 (M odd) or 1, 3, 5 . . . Mxe2x88x921, M, Mxe2x88x922, . . . 6, 4, 2 (M even). By addressing the strobe electrodes in this way, any brightness variation in a particular row of pixels will tend to be compensated by adjacent rows of pixels, since adjacent rows of pixels will be addressed at different times and so will be at different points along the characteristic curve of the transmissivity against time.
The combined duration of steps (a) to (d) may be substantially 16 msec and the duration of step (c) may be substantially 7 msec.
The display device may comprise a backlight, and the backlight may be on during all of steps (a) to (d).
The display device may comprise a backlight wherein the backlight is on for only part of a frame. The backlight may be off during step (a).
The device may be a pi-cell, and may have d/pxe2x89xa00.
A second aspect of the present invention provides a surface mode liquid crystal display device comprising: a liquid crystal layer having a non-zero twist angle; means for applying a first voltage having a first magnitude across a selected portion of the liquid crystal layer to put the selected portion of the liquid crystal layer into a first liquid crystal state; and means for changing a magnitude of the voltage applied across the selected portion of the liquid crystal layer while the brightness of the part of the liquid crystal device corresponding to the selected portion of the liquid crystal layer is greater than the equilibrium value of the brightness associated with the first liquid crystal state.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.